Optical discs such as compact discs (CDs), video compact discs (VCDs) and digital versatile discs (DVDs) are generally used to record date onto a data side thereof with a burner. For convenience, after the burning operation of a disc is finished, the title or other information should be marked on the label side of the disc. An approach for marking the disc utilizes a marking pen to write the information on the disc. If a wrong marking pen is selected, the disc is likely damaged because a very thin protective coating on the label side of the disc is vulnerable to chemical or physical attack. In accordance with another approach, an adhesive label is attached onto the label side of a disc. Although the attachment of the adhesive label makes the disc look more professional, there are still some drawbacks. For example, any air bubbles in the adhesive label may cause trouble. If the adhesive label is not perfectly aligned, vibration is likely rendered when the optical disc is rotating in the disc reading apparatus. For reading an unbalanced disc, the rotating speed has to be lowered in order to avoid errors.
Another method for drawing the label pattern onto the label side, which is also referred as a light-scribe technology, requires special discs with printable surfaces. The label sides of the discs are coated with a light-sensitive dye that becomes darkened when exposed to the laser light in a specially designed disc burner. After a blank disc is burnt in the usual way, the disc is flipped over and loaded to the burner again. By creating the desired label design on the computer system using a graphics program, the laser light burns the label pattern onto the label side. In comparison with the conventional disc marking method, the light-scribe technology is able to create high-quality label pattern on the disc, for example the effect of serigraphy or grey level.
Referring to FIG. 1, a conventional control system of an optical disc drive 100 is shown. An optical disc 110 is driven to rotate by a spindle motor 120. For reading data from the rotating disc, the optical pickup head (PUH) 10 is driven to move in the tracking direction by a sled motor 130 to perform a seeking operation. Further, the lens 1 of the optical pickup head 10 is driven to move in the tracking direction by a tracking coil 140 to perform a tracking operation or short seeking operation. The term “tracking operation” used herein means that the position of the lens 1 with respect to a selected track is maintained in the proper center position above the selected track. The term “seeking operation” or “short seeking operation” means that the lens 1 jumps from one track to another track. In addition, the lens 1 is driven to move in the focusing direction by a focusing coil 145 to perform a focusing operation.
When an electronic signal is generated responsive to an optical signal reflected from the optical disc 110 and received by the optical pickup head 10, the electronic signal is transmitted to a radio frequency (RF) amplifier 150 to be processed into a radio frequency signal RF, a tracking error signal TE and a focusing error signal FE. These signals RF, TE and FE are further processed by a digital signal processor (DSP) 170 to generate three control signals. In response to these three control signals, a first motor driver 160 makes adjustments to output driving forces for driving the sled motor 130, the tracking coil 140 and the focusing coil 145, thereby properly locating the optical pickup head 10 onto the desired track and desired focusing position. Under the control of the digital signal processor 170, a second motor driver 165 output a driving force for driving the spindle motor 120, thereby permitting rotation of the disc 110 at a revolving speed.
Unlike the data side of the disc, the label side for drawing the label pattern thereon according to the light-scribe technology has no tracks for the optical pickup head 10 to perform the tracking operation. Therefore, the optical pickup head 10 and lens 1 should be driven to move by the sled motor 130 and the tracking coil 140 in an open-loop control manner. Referring to FIG. 2, the label pattern printed by the optical disc drive supporting the light-scribe technology is illustrated. The tracks are determined by angular rotation of the sled motor 130. Due to the deviation resulted from the backlash of the driven gear, a pattern gap 20 between a first portion 21 and a second portion 22 is generated. Generally, the optical pickup head 10 is driven to move by the driven gear of the sled motor 130 from the inner track to the outer track in order to draw label pattern. For example, if the optical disc drive is preset to draw the label pattern from the 100th track to the 110th track and from the 200th track to the 210th track, the optical pickup head 10 is driven by the driven gear of the sled motor 130 to firstly draw label pattern on the region from the 100th track to the 110th track, then shift outwardly from the 200th track to the 210th track, and finally draw label pattern on the region from the 200th track to the 210th track. In a case that a command is issued to draw label pattern from the 111th track to the 120th track at that moment, the optical pickup head 10 will be driven by the driven gear of the sled motor 130 to firstly shift inwardly from the 210th track to the 111th track, and then draw label pattern on the region outwardly from the 111th track to the 120th track. Since the optical pickup head 10 is driven by the driven gear of the sled motor 130 to move inwardly to a selected track, the visible pattern gap 20 between the 111th track and the 110th track is generated due to the deviation resulted from the backlash of the driven gear.